The precoding matrices defined in the 3rd Generation Partnership Project (3GPP) wireless communication protocols and elsewhere are generally believed to yield a set of antenna patterns having equal energy. However, there is a contradiction in the specification in that if the precoding matrices are applied in a way that the resulting antenna patterns are equal in energy, the precoding matrix-based channel estimates used for demodulation, precoding matrix selection, and channel quality estimation will have scaling error. Furthermore, the scaling error will be a function of the precoding matrix. Conversely, if the precoding matrices are applied in such a way that the precoding matrix-based channel estimation is correct, then the resulting antenna patterns will not have equal energy and precoding matrix selection via feedback from the UE will not be optimal. In the selection of the precoding matrices in the 3GPP specification, it was assumed that the energy of the antenna pattern is equal to the energy of the precoding matrix. However, without normalization of the antenna patterns, it can be shown that for some precoding matrices, the energy of the resulting antenna pattern is significantly greater than the energy of the precoding matrix. Conversely, for other precoding matrices, the energy of the resulting antenna pattern is less than the energy of the precoding matrix. Without a model for energy gain or loss (e.g., ohmic heating, impedance mismatch) within the antenna array, the array pattern should be normalized such that the energy of the antenna pattern is equal to the energy of the precoding matrix for all precoding matrices in the codebook.
In 3GPP, the current method used to estimate the channel gain for the data assumes that no re-scaling of the antenna pattern is necessary to maintain unit energy for all precoding matrix indicators. If the data symbols are transmitted with equal energy for all precoding matrices and if no PMI-based renormalization is performed at the UE, the precoding matrix indicator/cell-specific reference symbol (PMI/CRS)-based channel estimate will be in error. In order to compute the optimal PMI—channel quality indicator (CQI) pair, the UE must be able to compute the complex channel that would result from the application of a given PMI. With the current model, the UE does not renormalize the pattern gain for each PMI and thus the corresponding channel gain estimate used to determine the CQI will erroneous. As a result, the wrong PMI-CQI pair may be sent to the UE. Further, even if the correct PMI is chosen, the corresponding CQI may be in error. The problems described above can be expected to be most significant when the eNB antenna array is closely spaced (e.g., half-wavelength spaced arrays). The normalization issue is less significant with diversity-spaced arrays but may still be problematic.
The various aspects, features and advantages of the invention will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.